In the field of connection elements which are mechanically attached to sheet metal parts on the production thereof, a distinction is made between press-in elements, on the one hand, and riveting elements, on the other hand. Press-in elements are characterized in that they are at least not intentionally deformed on attachment to a sheet metal part, but that rather the sheet metal part itself is deformed and brought into engagement with shape features of the press-in element, whereby the press-in element is fastened to the sheet metal part in a manner secure against rotation and against being pressed out. With riveting elements, the rivet section of the element is intentionally deformed on the attachment to the sheet metal part, usually to form a rivet bead, whereby the sheet metal part is captured between the rivet bead and a flange part in order also here to achieve a connection secure against rotation and against being pressed out.
Both press-in elements and riveting elements are furthermore known in the form of self-piercing elements. The name self-piercing is to be understood such that the corresponding element punches its own hole in the sheet metal part; naturally only when a sufficient force is exerted onto the self-piercing element, for example by a press, by a robot or by a power-actuated pair of tongs, which presses the self-piercing element against the sheet metal part, with the sheet metal part being supported on a corresponding die button on the side remote from the element.
It was previously usual in automotive construction to use self-piercing elements with sheet metal parts which have a strength below approximately 300 MPa. The self-piercing elements which are then used usually have a strength in the range between 700 and 900 MPa and, in exceptional cases, up to approximately 1250 MPa, which is absolutely sufficient to stamp out a hole in the sheet metal part on the application of the connection element to the sheet metal part, in particular when the sheet metal part has a thickness of less than approximately 3 mm. Strengths of the elements up to approximately 850 MPa apply, for example, to elements of class 8, whereas higher strengths apply to elements of classes 10 and 12 which normally make a heat treatment and/or a specific choice of material of the corresponding elements necessary.
In the connection elements used, the material of the connection element in the unprocessed state usually has a strength of approximately 380 MPa. This strength increases, however, to values in the range between 700 and 900 MPa solely due to the cold deformation which is used to generate a fastening element by cold heading starting from a bar material.
For some purposes, connection elements are required which have a higher strength than 900 MPa. They are then manufactured from a material which can be hardened by a heat treatment, whereby a higher strength is reached. Such heat treatments are, however, unwanted in many cases. They represent an alien process in comparison with the manufacture of the connection elements using cold heading and are usually not even carried out in the same factory in which the connection elements are manufactured, whereby a substantial expenditure of time and cost is involved in producing heat-treated elements of higher strength.
On the other hand, sheet metals having higher strength are increasingly being used in automotive construction and in some cases also heat-treated sheet metal parts which have higher strengths above the usual strength range from 700 to 900 MPa for connection elements. Such sheet metal parts having higher strength make it possible, on the one hand, to work with thinner metal sheets, whereby weight can be saved, but make it extremely difficult, on the other hand, to work with self-piercing elements.